The present invention relates to SPM cantilevers and fabricating methods thereof for use in Scanning Probe Microscopies (SPM) such as Atomic Force Microscopy (AFM).
Scanning Probe Microscopies (SPM), apparatus having an atomic-order measuring resolution such as for use in measuring surface irregularities, are now widely used. In recent years, however, measurements at yet higher resolutions are demanded. For this reason, it is desirable for the cantilever used in SPM to be provided with a probe portion sharpened at its terminal end and at the same time having high aspect ratio.
Among such SPM cantilevers is a cantilever as disclosed in Japanese Patent No.2624873. The SPM cantilever uses a silicon oxide film as the lever material. In particular, a resist film is formed on the surface of a thermally oxidized silicon film having a thickness of 1 to 2 μm and a sharpened portion is formed in the resist film by using photoetching technologies. A silicon oxide film probe portion having a smaller radius of curvature than the sharpened portion formed of the resist film is then formed by effecting an isotropic etching by a buffer etching solution. With such fabricating method of SPM cantilever, an SPM probe portion can be formed as having a radius of curvature of 0.1 μm or less at its terminal end by using the conventional photoetching technologies and also as having favorable adhesion between a cantilever support portion and the lever by using the thermally oxidized silicon as the lever material.
Further, a processing method of cantilever using focused ion beam (FIB) has been disclosed in Japanese Patent No.2984094. In the processing method, a focused ion beam is caused to irradiate a probe's terminal end portion with arbitrarily changing the scanning direction so as to process the probe terminal end portion into a sharpened form. With such processing method of cantilever, it is possible to arbitrarily change the vertical angle at a terminal end of the probe and to vary an aspect ratio thereof. At the same time the radius of curvature of the terminal end of the probe can be processed into 50 nm or less.
The previously suggested cantilevers as described above, however, have problems as follows. First, a problem in the cantilever disclosed in Japanese Patent No.2624873 is that it becomes impossible to accurately ascertain the surface irregularities of a sample, even though the terminal end of the probe can be sharpened. This problem will be described below with reference to FIG. 1. FIG. 1 shows the relation between the probe's terminal end and a sample to be measured, including: a lever portion 101; a plate-like probe portion 102 of silicon oxide film formed on a free end of the lever portion; and a sample 103 to be measured. Supposing as shown in FIG. 1, L1 as the plate thickness of the probe portion and L2 as the length of the sharpened terminal end portion formed by etching in the case where an isotropic wet etching of the plate-like silicon oxide film probe portion 102 is effected by using a resist mask, L1 and L2 are equal to each other in length or, more likely, the relationship of L1>L2 holds due to the fact that the terminal end portion is etched away also from sides of the resist mask. Accordingly, length L2 of the probe terminal end portion to be sharpened becomes shorter. If this is used in measuring the sample 103 which has relatively large irregularities, those portions other than the probe's terminal end are brought into contact with the irregularities of the sample 103 to be measured so that it becomes impossible to accurately ascertain configuration of the irregularities.
Also, since the silicon oxide is etched away in a short time period by fluoric acid, the fabrication of a sharpened probe portion of the order of nanometers with controlling variance thereof is difficult due to the variance in etching if a plurality of cantilevers each having probe portion are to be fabricated within a wafer.
Of the cantilever disclosed in Japanese Patent No.2984094, on the other hand, though the forming of a probe portion having high aspect ratio is possible, the radius of curvature of the terminal end of the probe portion is difficult to be regulated to result a lower yield, since FIB processing must be performed for each one cantilever. Also, in the case of a thin lever portion, warping of the lever might be caused due to damage or heat in the FIB processing. Further, in addition to the expensiveness of FIB apparatus, there is a problem of increased cost for example because of the excessively long time to be consumed in forming several hundred cantilevers on a wafer.